Another world inside of a portal issues
Ok this is a tutorial on how to create a portal using scenes, render layers, object id and compositing. Please try to achieve it while reading the tutorial.

First you want to have the first world and the portal.



Note that i set the plane pass index to 1, this is crucial.

Then you need the second world, I did a terrain with a sunlamp.
Create them in the second scene and link your camera here afterward (ctrl+L-&gt;link to scene).



After this you need to set up the render layers, for use with compositing.
Go to render layer panel, unfold the pass menu and select object index



For the scene 2, you don't need to set the object index because the plane you will be using for splicing the two world is only present in the first scene. 

Everything is setup, open the node editor and select the compositing nodes.

Here you start with a render layer that is set on render layer one, scene one (we are just using one scene in our case) and a composite output.

Just add another render layer, set in on scene 2, add an alpha over node and connect your two layers color output on that node, the factor is the plane, so connect the indexOB that we activated earlier for the first layer to an IDMask node then to the factor of the alpha over. Easier to just look at the picture here:



Now you can just render.



Being an after effect, there is no such thing as compatibility issues regarding wether you are using cycles, eevie, or blender internal.